The present invention relates to environmental protection of junctions in elongate substrates, such as splices in cables, particularly in telecommunications cables.
It is frequently necessary to protect such junctions against the environment in order that the cables or other substrates may continue to function properly. Protection generally has to be provided against moisture, corrosive chemicals as well as insect and animal damage etc. The intention when enclosing a junction such as a cable splice is to make good the original cable insulation that had to be removed in order to connect the conductors, and it is generally required that the life-time of the seal provided by the new enclosure be comparable to that of the original cable insulation. It will be appreciated therefore that the material of the enclosure must provide a highly resistant barrier for a considerable period of time.
One way of providing such a barrier is to install around the cables a splice case comprising an imperforate sleeve of a modified polyolefinic material in conjunction with a high performance adhesive. Such sleeves are conveniently produced by extruding a continuum of material. The sleeve is preferably made recoverable so that it can be shrunk (or otherwise recovered) into close contact with the cables.
There is a further consideration relevant to the design of enclosures for cable splices, and that is the ability to retain pressure. Many types of cables and splice cases are pressurised during use, are assessed in terms of pressure retention to determine their quality, or become subject to incidental pressurisation during use. The importance of this consideration is of course different in each of these three situations, but it is accepted that the ability to retain some degree of pressure is a necessary feature of a splice case if environmental protection is to be achieved.
The most stingent requirements are for a splice case for pressurised cables, such as main cables in a telecommunications system. These cables are pressurised to prevent ingress of water in the event of damage and to provide a means of fault detection. Here the product must withstand a pressure of the order of 10 psi (70 kPa) throughout its life, and a functional test designed to mirror such long term performance requires impermeability at, say, 70 kPa over 10 eight hour cycles between -40.degree. C. and +60.degree. C. in air (Bell cycle). An alternative cycle is in water over four hours at 105 KPa between 5 and 50.degree. C. In addition to this cyclical environmental test, the product may be tested for integrity by pressurisation at 150 kPa in water for about 15 minutes at 23.degree. C. No leak should be observable. A product that is to operate continuously at pressure should also possess long term creep resistance if it is not to become significantly distorted during use.
In telecommunications distribution cables, for example, an ability to retain pressure is required as an indication of completeness of environmental sealing, although the cables are not pressurised during use. Various temperature/pressure cycles have been devised for this purpose, and one that is preferred is a modified Bell Cycle which involves temperature variation from -40.degree. to 60.degree. C. over 8 hours at an air pressure of 40 kPa. The splice case should show no leak after 10 cycles. An alternative cycle is a temperature variation between room temperature and 70.degree. C. at a pressure of 105 KPa over 4 hours.
These and other cable splice cases may become pressurised through being exposed to sunlight, or through the heat involved in the last stages of heat recovery when the seals to the cable have been formed. In such cases it is necessary that the splice case be able to maintain this temporary, and generally rather low, pressure if the environmental seal is not to fail.
Many of today's splice cases for pressurised cables are large and heavy, and consist of many components. For example, cast iron case halves are bolted together around the cable splice, the cable entries being sealed by a complex arrangement of compression collars, clamps, sealing washers and tape. Variations on this system exist but there remains the problem of sealing the cables to the splice case at their points of entry. A large stock of parts must be kept if various sizes of cables are to be joined, or if the number of cables per splice case is likely to vary. A further problem is that installation is difficult and lengthy. The problems associated with such multi-part, rigid, splice cases are avoided by the use of recoverable sleeves: installation is quick, and a variety of sizes and numbers of joined cables can be enclosed with a small number of parts. The use of a continuum of a suitable polymeric material, together with an adhesive can provide excellent environmental sealing and pressure retention. The sleeve is preferably used in conjunction with a liner which surrounds the cable splice and underlies the sleeve. The liner provides mechanical strength, gives the splice case its shape, facilitates re-entry, and may protect the conductors from damage during heat recovery.
However, in unfavourable circumstances and where pressure retention is a primary design consideration, it may be thought desirable to increase the wall thickness of such recoverable polymeric sleeves in order to ensure no movement or creep over long periods of time. A greater wall thickness unfortunately makes the product more difficult and thus more costly to manufacture, due to cost of material and to problems in cross-linking and expanding the material. Also, heat-shrinkage of a thick-walled product takes longer, and requires a more careful application of heat if damage to the cables or other substrates is to be avoided.